Environment Variable Resolution? about compose

[aanand]

@jamestenglish That would be cool, but the environment variables added by links are pretty unnecessary anyway, since:

1. An entry is added to /etc/hosts with the name of the link (in this case, db)
2. The port given to web will always be the one exposed by db, (in this case, 3306).

(This is why we no longer recommend using the environment variables provided by links.)

You can therefore just write:

environment:
  DATABASE_URL: mysql://user:password@db:3306/db

[bfirsh]

Yes! We were talking about this yesterday and I would really like this to work. I haven't thought a whole lot about how it should be implemented though – it's a little complex because the env variables aren't available until the container has been started (I think).

Maybe we could run a one off container that simply runs env to get the environment the container is going to see, then use that to set extra custom environment variables.

[SkinyMonkey]

As you said the environment variables are not available until you run the container.
The thing is that environment variables injected by docker, like the one for the links, as in your exemple :

"DB_1_PORT_3306_TCP_ADDR"

will not be the same between two instances of db, even if they have the same name.
For example here the IP won't be the same, except if you change the network settings (i assume, never tried though)

I thought that you could do this:

db:
     image: example/mysql

test:
      links:
            - db
      environment:
            - MYSQL_USER=root
            - MYSQL_ROOT=toor
            -DATABASE_URL=mysql://$MYSQL_ROOT:$MYSQL_PASSWORD$DB_1_PORT_3306_TCP_ADDR/db

But Docker is simply injecting the environment variables without expanding them ... too bad.
That could be a feature to add directly to docker though.

[sherzberg]

@SkinyMonkey i agree this would be a great addition to docker. are you familiar with the code? any idea where to start poking?

@bfirsh until then, any thoughts on adding this here? id love to send a pull request, but not sure where in your code is the best to do this? i see a few places in container and service where it could live. if you have thought about this, let me know where to start i can give it a shot. maybe a proof of concept here can vet out some ideas for attempting to get this in docker itself.

[thestonefox]

Hi,

Could this be extended to being able to reference environment variables from the host machine to be injected into the docker container via fig?

e.g.

environment:

• DB_PASSWORD=$DB_PASSWORD <--this being an env variable on the host machine

[aanand]

@thestonefox Yep, useful feature. See #76 - still under discussion.

[bajohns]

Greetings - looks like the discussion might be resolved - any chance this will make it into the next release?

Thanks!

[aanand]

Closed by #297.

[pnegahdar]

@aanand I don't think this was actually close by #297, the issue in question here is reassigning autoassigned env vars to another env var (like one the codebase already expects), i.e an alias. 297 seems to allow blank envs to resolve to the host env.

[jamestenglish]

@aanand I agree with @pnegahdar the original use case:

web:
  build: .
  ports:
   - 8000:8000
  links:
   - db
  environment:
   DATABASE_URL: mysql://user:password@${DB_1_PORT_3306_TCP_ADDR}/db
db:
  image: orchardup/mysql

Does not seem to be supported by #297

specifically how would one be able to set an environment variable based on other environment variables like this line: DATABASE_URL: mysql://user:password@${DB_1_PORT_3306_TCP_ADDR}/db

I think this is an important use case as apps should not have to be burdened with changing code to support esoteric environment variable names like: DB_1_PORT_3306_TCP_ADDR

[jamestenglish]

@aanand Ah thanks, I had been out of the docker world for a little bit and didn't realize they added that feature

[dimroc]

Thanks a lot @aanand, your approach is perfect.

[kelvv]

@aanand Thank you , perfect